Maintenance of General Anesthesia
🎯 Anesthetic goals (ช่วง maintenance)
- Maintain
Stage III (surgical anesthesia):
- Unconsciousness
- Amnesia
- Immobility
- No
response to noxious stimulus
- ควบคุม physiology:
- Oxygenation
/ ventilation
- Hemodynamics
- Temperature
- ป้องกัน awareness with recall
- ให้ analgesia + muscle relaxation ตามความจำเป็น
⚙️ Techniques
1.
Inhalation anesthesia
2.
Total intravenous anesthesia (TIVA)
3.
Balanced / multimodal anesthesia (นิยมมากที่สุด)
👉 ใช้ combination
→ synergistic effect → ลด dose แต่เพิ่ม risk hypotension / delayed emergence
🎚️ Depth of anesthesia
- Titrate
individual drugs → หลีกเลี่ยง too deep → hypotension / neurocognitive disorder
- Monitoring:
- ETAC
- EEG
/ BIS
- Target:
- ~ 0.8–1
MAC → ลด awareness
🌬️ Inhalation anesthesia
🔑 Key concept: MAC
- 1
MAC = prevent movement in 50%
- Influenced
by: age, comorbidity
💊 Volatile agents
|
Agent |
จุดเด่น |
ข้อเสีย |
|
Sevoflurane |
induction + recovery เร็ว |
cost สูง (long
case) |
|
Desflurane |
fastest onset/offset → obese/OSA ดี |
airway irritation, tachycardia |
|
Isoflurane |
cheap, long case |
recovery ช้า |
|
Halothane |
cheap, potent |
hepatotoxicity, arrhythmia |
✅ Advantages (volatile)
- titration
ง่าย
- bronchodilation
- muscle
relaxation
- monitor
ETAC ได้
❌ Disadvantages
- respiratory
depression
- vasodilation
→ hypotension
- ↑ PONV
- malignant
hyperthermia trigger
💨 Nitrous oxide (N₂O)
- ไม่ใช้เดี่ยว (MAC 104%)
- ใช้เป็น adjunct
ข้อดี
- rapid
onset/offset
- analgesia
- ↓ requirement agent อื่น
ข้อเสีย
- expand
air spaces → ห้ามใช้ใน:
- pneumothorax
- bowel
obstruction
- intracranial
air
- diffusion
hypoxia
💉 TIVA (Total Intravenous
Anesthesia)
🔧 Core components
- Hypnotic:
Propofol
- Analgesic:
Opioid (e.g., remifentanil)
📌 Indications
- malignant
hyperthermia risk
- high
PONV risk
- neuromonitoring
(SSEP/MEP)
⚖️ Pros
- ↓ PONV
- control
depth (via infusion)
- better
neuromonitoring compatibility
⚠️ Cons
- ไม่มี real-time concentration (เทียบ ETAC
ไม่ได้)
- risk
awareness
- infusion
error / IV infiltration
- hypotension
/ respiratory depression
💊 Key IV agents
🧪 Propofol
- Dose:
75–150 mcg/kg/min
- Pros:
- rapid
recovery
- antiemetic
- Cons:
- hypotension
- respiratory
depression
💉 Opioids
- Short:
fentanyl
- Infusion:
remifentanil
- Long
(post-op pain): morphine / hydromorphone
Side effects
- respiratory
depression
- PONV
- delirium
- hyperalgesia
🧩 Adjuvants
|
Drug |
Benefit |
Risk |
|
Dexmedetomidine |
opioid-sparing, sedation |
bradycardia, hypotension |
|
Ketamine |
analgesia, maintain airway |
↑
BP/HR, hallucination |
|
Lidocaine infusion |
analgesic adjunct |
LAST |
|
Remimazolam |
ultra-short benzo |
hypotension, rare anaphylaxis |
💪 Neuromuscular Blocking
Agents (NMBA)
📌 Role
- muscle
relaxation
- improve
surgical condition
⚠️ Risks
- awareness
↑ (no movement)
- residual
paralysis → morbidity
- anaphylaxis
(rocuronium, SCh)
🧪 Types
- Depolarizing:
Succinylcholine
- Nondepolarizing:
rocuronium, vecuronium, etc.
🔚 Emergence (overview)
- discontinuation
anesthetics → return
to consciousness
- ต้อง plan:
- analgesia
- reversal
NMBA
- avoid
delayed emergence
🧠 Key Clinical Pearls
- Balanced
anesthesia = best practice
- Avoid
excessive depth + hypotension
- ETAC
+ BIS ช่วยลด awareness
- TIVA:
- ดีใน PONV / neuromonitoring
- แต่ต้องระวัง infusion error
- N₂O:
- useful
adjunct แต่ต้อง screen contraindications
- NMBA:
- ต้อง monitor →
ป้องกัน residual block
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